How to confirm a blown intake manifold plate on a Dodge V-6 or V-8 engine.

This test procedure is very helpful when trying to tune a Dodge truck or van that is operating on Propane or Natural Gas, either dedicated or bi-fuel (can operate on either Propane / Natural Gas, OR, gasoline.)

 

Symptoms:

The vehicle may have exhibited a backfire, then a stumble when operating on the alternative fuel.  Gasoline operation MAY be normal, or near normal.  Operation may or may not be accompanied by a high pitch squeal that is hard to locate.

This manifold plate had blown off the bolt heads of 8 of the 15 bolts!  The front seal had blown out, the valve covers were significantly damaged and had to be replaced, the mixer was destroyed, the vaporizer needed a rebuild, and the air breather box was destroyed.  It also blew the dipstick tube out until it hit the hood.  There was no engine damage and all the bolt heads were captured by the camshaft gallery.

This vehicle had 40,000 miles on the conversion, by a prominent conversion shop, and had all the correct equipment installed, with one exception.  See below for details.

What causes the backfire?

This engine design has a very large volume intake manifold that is shaped like a nautilus shell.  Air enters the upper portion, near the center of the manifold.  Air is then directed down to the bottom, then allowed to flow along a flat plate (approximately 6" wide X 14" long).  Intake runners then pick up the air stream from this plate, allow the air to flow up, then turn 180º, then into the cylinder head.  Total intake runner length from the bottom of the intake runner to the intake valve may exceed 20".

The length of the intake air flow from the bottom of the throttle body to the beginning of the runners may be as long as another 14".  Add in the distance from the mixer inlet (or the point where the fuel is mixed with the air) and you have another 6 to 24 inches!

The accumulated distance can be as much as 40 to 54 inches!  This is a great deal of air and fuel to move between fuel mixture changes accomplished by the vaporizer-regulator.

Remember that all fuel mixture changes are performed at the vaporizer regulator, using a conventional Impco or OHG system.  Other systems with a venturi mixer have the same obstacle, but usually have the air fuel mixer directly mounted to the throttle body.

The process of combustion proceeds as follows:

  • Air and Fuel is metered into the combustion chamber
  • The mixture is ignited, then allowed to flow into the exhaust manifold
  • The burnt mixture reaches the O2 sensor, where the amount of residual oxygen is measured
  • An electrical signal is sent to the fuel mixture controller, where a calculation is made
  • The fuel controller sends an electrical signal to the vaporizer-regulator, where an electrical vacuum solenoid is triggered
  • A varying amount of air-valve vacuum is commanded, to either release vacuum to allow the vaporizer-regulator to allow more fuel to enter the engine, or to apply more air-valve vacuum, to reduce the amount of fuel enter the engine
  • Fuel is then allowed to flow from the vaporizer-regulator to the air valve mixer where it is mixed with incoming air
  • The mixture is allowed to flow from the mixer-through the throttle body-through the intake manifold- then into the cylinder head, where the process is repeated.
This process is called the "Fuel Transport Delay", or the amount of time (measured in crankshaft degrees of rotation).  In some cases, the crankshaft may rotate as many as 20 revolutions before the cylinder ingests the new changed air-fuel mixture!
This author has seen mis-matched spark plugs, improperly gapped plugs, crossed secondary spark plug wires, no mixer mounting gaskets, loose mixer adapters, O2 sensors that are "flat line" (burned out), throttle body gaskets leaking, and broken valve springs.  All of these vehicles exhibited backfires that were blamed on the use of alternative vapor fuels!
Assuming that all tune and mechanical problems are properly serviced or repaired, backfires are most frequently caused by a lean air-fuel mixture, usually encountered when the engine is started with an open throttle, but can also be caused by a tip in (throttle stab).

Diagnostic steps:

Connect the Autotronics Controls pn #4041, and read the air fuel mixture and duty cycle of the fuel solenoid.

The fuel mixture solenoid should be fluctuating at approximately 50%, or ranging from 5 to 9 volts.  The O2 sensor should be cycling between the green and red lights.

If the fuel mixture is locked full lean, attempt to enrichen the mixture by gently depressing the primer button on the vaporizer.  If a gentle pressure is all that is required to bring the fuel metering solenoid into spec, attempt to make an adjustment to the idle mixture.  If more than 1 turn of a mixture screw (with the Impco 300-A) or 3 or more turns (with the 200-225 or 425) is required to bring the fuel mixture into range, suspect a significant vacuum leak.
It makes no difference which fuel the engine is operating on, at this time.  Be sure the engine is at correct operating temperature.
  • Remove the PCV valve from the valve cover, plug the opening with a suitable plug.  Do not force it in, the rubber grommet is usually flexible enough to allow a plug to be inserted
  • Remove the crankcase breather from the opposite valve cover
  • Place your hand over the opening in the valve cover
  • No vacuum should be felt there, period!
  • If the engine is operating on the alternative fuel, you might notice a rich engine fuel mixture at this time.
If ANY vacuum is felt, that means that a vacuum leak exists in the engine, below the intake manifold.  With the engine operating normally, a slight amount of pressure should develop with combustion blow-by.
Follow accepted industry procedures and remove the intake manifold.  Invert the manifold and remove the plate and inspect the gasket.  Generally a significant section will be either blown out or drawn into the manifold.  Also, while the plate is off, inspect the manifold and notice the length of the runners!
Clean all mating surfaces.  Use a degreaser on the plate and manifold flanges.  Apply a light bead of silicone sealer (RTV).  The bead should be no larger than 1/8" (3 mm).  (My favorite RTV is the high bulk sealer usually used by the OEMS for assembly, but the Ultra Blue is excellent.)

Reassemble following industry standards.
After the engine is reassembled, run the engine and verify the fuel mixture settings.


Install an RCV in the vaporizer cover.  This is a VERY important step!  

Either:

  • Remove the black plastic elbow and install a tee fitting, then reinstall this RCV with the opening pointing to the rear, or pointing down;
  • Remove the cover, drill a hole to accept a 1/8" NPT fitting, deburr the threads, then reassemble.

Earlier, I mentioned the high pitch squeal.  This is usually caused by the front timing cover seal vibrating, like a clarinet reed, from the air being pulled through it by the vacuum present in the oil pan!
   

The views expressed in this forum are the authors only and do not represent any organization or manufacturers opinion.  The author has encountered this problem several hundred times in the last 6 years and have found this problem wide-spread.  See the home page for related problems with Ford and GM vehicles.

Closed document copyright reserved.

Franz Hofmann